<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /*  视口大小为200px时css样式生效*/
        @media(width:200px){
            h1{
                background-color: red
            }
        }
        /* 800以内为绿色，最大不超过800，超过就不生效 */
        @media(max-width:800px){
            h1{
                background-color: green;
            }
        }
        /* device-width 电脑分辨率大小 */
        @media(device-width:1537px){
            h1{
                font-size: 1000px;
            }
        }
    </style>
</head>
<body>
    <h1>123</h1>
</body>
</html>